Phosphate Rock: Using Biological Processes to Increase Its Effectiveness as a Fertiliser

Keywords: Crop rotation, organic acids, mycorrhizal fungi, phosphate solubilisers, composting, fermentation.

Balancing the agricultural phosphorus (P) cycle is difficult, particularly for arable/horticulture based systems, where crop products are exported off-farm. Sustaining soil P concentrations usually relies on the input of some form of P. Fertiliser products vary from those that are essentially unaltered to those that have undergone a high degree of processing to increase P solubility and enrichment. One of the major drawbacks associated with the direct use of P rock is its poor solubility in circum-neutral and calcareous soils, which requires long-term and unnecessarily large application rates in order to satisfy crop demand. Opportunities exist whereby the solubility and therefore bio-availability of sparingly soluble rock-P can be enhanced without the recourse to energy demanding processes. As well as a range of rock and soil factors, a range of crop factors also interact in governing the effectiveness and efficiency of P. A range of root traits have been shown to be beneficial in improving P efficiency of different crops and varieties. These include: root properties (morphology and root hairs), uptake kinetics parameters and root-induced rhizosphere changes (pH, organic acids and acid phosphatase). These responses have a direct benefit for the crop itself, but may also have benefits for the crop rotation as a whole, if P is mobilised from recalcitrant soil P fractions and mobilised P is made available to following or adjacent crops. The mineralisation of organic matter by soil micro-organisms provides an important supply of available P for plant growth and hence P supply maybe related to microbial activity in soil. In addition, associations between plant roots and arbuscular mycorrhizal fungi are able to use the available forms of P in the soil solution more effectively than roots alone. A range of soil micro-organisms including fungi and bacteria have the ability to solubilise mineral forms of P in culture and there is some evidence that this might also occur in soil. Inoculation of soil or seed with P solubilisers has not been a commercial success to date. Approaches which apply P solubilising micro-organisms to PR ahead of application to the soil have therefore also been considered. Both composting and fermentation approaches have been tested but despite the success of biotechnological solubilisation of phosphate rock at a small-scale, a usable technology at field or farm scale has not yet been produced. There are a range of opportunities to harness biological processes to increase the effectiveness of PR as a P fertiliser in agricultural systems. These approaches are not mutually exclusive and the most effective strategy is likely to seek to draw on the potential of them all within the economic and technical constraints of the farming system. However, there is a need to increase the understanding of the links between soil ecology and function to enable robust management advice to be derived.

E A Stockdale, School of Agriculture, Food and Rural Development, Newcastle University, Newcastle-upon-Tyne, NE1 7RU, UK, and

C A Watson, SAC, Craibstone Estate, Aberdeen, AB21 9YA, UK, and

A C Edwards, Nether Backhill, Ardallie, by Peterhead, AB42 5BQ, UK.

24 pages, 4 figures, 3 tables, 91 references.